The trend towards high-efficiency, environment-friendly drive systems for motor vehicles means that sufficient waste heat for heating the vehicle interior is no longer available. Compared to vehicle engines, particularly, the drive train of an electric vehicle works much more efficiently. Due to less waste heat and the lower temperature level of the refrigerant circuit that cools the drive train, the comfort conditions worsen in vehicles with the heating of the vehicle interior exclusively based on the engine refrigerant circuit. To meet familiar comfort demands requires additional measures. Therefore, efforts are made to heat the interior of an electric vehicle using, for example, an electric resistance heating system based on the use of so-called high-voltage PTC or low-voltage PTC (PTC—positive temperature coefficient). But the additional electric heating has effects on the efficiency of the electric vehicle, particularly the HVAC system. The range of the vehicle is dramatically reduced.
In prior art, it is known to utilize for heating the vehicle interior, apart from the above mentioned additional heating systems, the refrigerant systems, or refrigerating plants, respectively, present in the motor vehicles. This can be realized, for example, by that for heating, the refrigerating plant is operated as heat pump.
Alternatively, a short circuit without secondary heat pickup can be used. Here, the mechanical driving power of the compressor is to a large extent transformed into heat for heating the vehicle interior.
Upon operation, the battery cells of the battery, as well as other components of the electric drive train such as the electric motor and the power electronics, heat up. Particularly, during decharging and charging, the battery should be operated at the optimal temperature. The developed and released heat is to be dissipated, because an increased temperature of operation causes a very high thermal load of the battery cells. Because of the limited temperature stability of the batteries, they have to be actively cooled. The ambient air, air of the vehicle interior, refrigerant, and coolant are suitable mediums to cool the battery and other electronic components of the drive train that also have to be protected from too high thermal loads, therefore to be thermally conditioned. For example, water or glycol, respectively, are used as coolant. Thermal conditioning of the battery and other components of the electric drive train is necessary both during travel as a dynamic operation and at a rest as a stationary operation of the vehicle.
Cooling the battery, which for a refrigerant circuit is to be considered to be a heat source, results in an increased life and should be performed such that the temperatures of the cooled battery only vary within a limited range. For an operation at an optimal operating temperature of the batteries of the electric vehicles, not only the developed heat is to be dissipated but also, at a too low ambient temperature, heat is to be supplied to the cold battery, especially during starting.
From prior art, HVAC systems of electric vehicles are known that first, serve to temper the components of the electric drive train and second, transfer absorbed heat to the air supplied into the vehicle interior.
DE 199 30 148 A1 discloses a system for the temperature control of the interior of a motor vehicle driven by an electric motor, with the energy provided to the system by so-called high-temperature batteries. A refrigerant circuit comprises a primary circuit with an external and an internal heat exchanger, wherein the internal heat exchanger is arranged in the path of a first air mass flow that is oriented in the direction of the interior. Further, the refrigerant circuit is provided with a secondary circuit with a compressor and a switching device for the direction of circulation of the refrigerant in the primary circuit. Thus, the system is operatable in a heating or cooling mode for the interior. A water cooling circuit making possible the cooling of batteries includes a liquid/air heat exchanger in the path of the first air mass flow and a liquid/refrigerant heat exchanger in the secondary circuit of the refrigerant circuit. The refrigerant circuit, in each case, has a heat source and a heat sink. If the refrigerant circuit is directly thermally connected to the water cooling circuit, the refrigerant circuit comprises an additional heat exchanger as a further heat source.
In DE 102 01 741 A1, a process for the tempering of a vehicle as well as an HVAC system and a heat source within the vehicle are provided, both supplied over a medium circuit with a common medium for cooling and/or heating. The medium circuit is provided with means for expanding and compressing the medium. Cooling of the heat source and air conditioning of the vehicle are realized by one medium. In addition to the heat source, the HVAC system includes heat exchangers that dependent on the demand can be operated as heat sources and/or heat sinks.
In both the system disclosed by DE 199 30 148 A1 and the HVAC system described in DE 102 01 741 A1, the heating function for the vehicle interior is realized over a heat exchanger that in cooling mode of the plant is switched as an evaporator. When this heat exchanger is afterwards used for heating again, a highly undesirable effect dangerous in relation to safety arises.
Particularly while the refrigeration system is being used as refrigeration plant, that is in a cooling mode, the evaporator placed in the ventilation system of the vehicle will dehumidify the air to be cooled. The humidity condensed on the evaporator surface is then, after a standstill of the vehicle and when started again with the heat exchanger that previously was operated as evaporator, used in heating mode as a condensor due to heating delivered to the air flow to be supplied into the interior. Such an alternating use of the refrigerant circuit as a refrigeration plant or a heat pump is more frequent in the transitional period.
The high humidity content in the air supplied to the vehicle interior causes precipitation at the cold surfaces in the vehicle interior, particularly, fogging of the window glasses with accompanying worsening of the passengers' vision. This effect is known as flash fogging.
A further developed system is, for example, disclosed by DE 10 2009 048 674 A1, wherein an HVAC system for a motor vehicle, particularly an electric vehicle or hybrid vehicle, with a refrigerant circuit is described that comprises an evaporator and a heat exchanger. The heat exchanger enables heat energy that is supplied into the refrigerant circuit to be fed into the fresh air of a passenger compartment. The HVAC system further is provided with an exhaust air line through which the air can be exhausted from the passenger compartment. The evaporator of the refrigerant circuit is in at least a partial area located in the interior of the exhaust air line. Therefore, the heat energy of the exhaust air is transferable into the fresh air in a heat pump mode of the HVAC system. Thus, the HVAC system is provided with components for heat recovery. The heat dissipated from the exhaust air is transferred into the fresh air by means of an intermediate medium and hence, additional heat exchangers.